CONCEPTS IN BIOLOGY

An understanding of how Earth formed and what conditions were like on early Earth will help us think about how life may have originated.

The "Big Bang"

Astronomers note that the current stars and galaxies are moving apart from one another. This and other evidence has led to the concept that our universe began as a very dense mass of matter that had a great deal of energy. This dense mass of matter exploded in a “big bang” about 13 billion years ago, resulting in the formation of simple atoms, such as hydrogen and helium. When huge numbers of these atoms collected in one place, stars formed. Stars consist primarily of hydrogen and helium atoms. The light from stars is the result of nuclear fusion, in which these atoms combine to form larger atoms. According to this theory, all of the chemical elements were formed as a result of nuclear fusion.

The Formation of the Planet Earth

Many scientists believe that Earth—along with other planets, meteors, asteroids, and comets—was formed at least 4.6 billion years ago. The protoplanet nebular model proposes that the solar system was formed from a large cloud of gases and elements produced by previously existing stars (figure 19.4). The simplest and most abundant gases were hydrogen and helium, but other, heavier elements had been formed by nuclear fusion and were present as well. A gravitational force was created by the collection of particles within this cloud, which caused other particles to be pulled from the outer edges to the center. As the particles collected into larger bodies, gravity increased and more particles were attracted to the bodies. Ultimately, a central body (the Sun) was formed, with several other bodies (planets) moving around it. Like other stars, the Sun consists primarily of hydrogen and helium atoms that are being fused together to form larger atoms, with the release of large amounts of thermonuclear energy.

FIGURE 19.4. The Formation of Our Solar System

(a) As gravity pulled gases and other elements toward the center, the accumulating matter resulted in the formation of the Sun. (b) In other regions, smaller gravitational forces caused the formation of the Sun’s planets. (c) Finally, the solar system was formed.

Thermonuclear reactions also occurred as the particles became concentrated to form Earth. Thus, the early Earth would have been a very hot place. Geologically, this period in the history of Earth is called the “Hadean Eon.” The term Hadean means “hellish.” Although not as hot as the Sun, the material of Earth formed a molten core, which became encased by a thin outer crust as it cooled. (How Science Works 19.1). In Earth’s early stages of formation, about 4 billion years ago, there probably was a considerable amount of volcanic activity.

Conditions on the Early Earth

The high temperature of the early Earth would have prevented an atmosphere from forming, because light gases such as hydrogen and helium would have escaped into space. Over hundreds of millions of years, Earth is thought to have changed slowly. As it cooled, volcanic activity would have released gases, and an atmosphere would have formed. Studies of current volcanoes show that they release water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nitrogen (N2), ammonia (NH3), hydrochloric acid (HCl), and various sulfur compounds. These gases would have formed a reducing atmosphere—an atmosphere that did not contain molecules of oxygen (O2). Any oxygen would have quickly combined with other atoms to form compounds. Further cooling enabled the water vapor in the atmosphere to condense into droplets of rain, which ran over the land and collected to form oceans.

HOW SCIENCE WORKS 19.1

The Oldest Rocks on Earth

Determining events that happened in Earth's distant past are difficult to achieve. The early Earth would have been molten and, as it cooled, the outer layer of Earth would have solidified into a thin crust of igneous rock. Earth is not static—it consists of geologic plates that are moving across the surface and in many places are being forced below the surface where they melt to reemerge at some other place. Thus, finding really old rock is rare.

In September 2008, scientists at McGill University in Montreal, Canada, announced they had found, on the eastern shore of Hudson Bay in northern Canada, the oldest Earth rocks yet discovered. To determine the age of the rocks they compared the amounts of specific isotopes of the element neodymium. This analysis led them to conclude that the rocks are about 4.28 billion years old. Since Earth is thought to have originated about 4.57 billion years ago, these rocks would have formed about 290 million years after the origin of Earth. These rocks are presumed to be some of the first formed on Earth as it cooled. The age of these rocks also suggests that Earth cooled faster than previously thought.

As with all new discoveries, other scientists are being cautious about accepting this new information. They would like to see additional evidence to confirm the date.

19.3. CONCEPT REVIEW

6. How did the atmosphere, the temperature, and the surface of the newly formed Earth differ from what exists today?